$ \left(\dfrac{25}{36}\right)^{\frac{1}{2}}$
Explanation: Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{25}{36}$ Figure out what goes in the blank: $\Big({\dfrac{5}{6}}\Big)^{2}=\dfrac{25}{36}$ So $\left(\dfrac{25}{36}\right)^{\frac{1}{2}}=\dfrac{5}{6}$